A. Field of the Invention
The invention relates generally to field-effect transistors and, more specifically, to chemically sensitive field-effect transistors for monitoring chemical concentrations.
B. Description of the Prior Art
During the past decade, a general recognition that semiconductor devices may be used to enhance chemical-sensing devices has evolved. In particular, there has been a recognized need to develop enhanced means of in vivo monitoring of certain ion concentrations such as potassium in the blood. One of the earliest attempts to use a semiconductor device as an ion-sensitive device is described in U.S. Pat. No. 4,020,830 to Johnson et al. The background of the invention section of the Johnson et al. patent describes a very early attempt by Bergveld to use a metal oxide semiconductor field-effect transistor (MOSFET) which was modified by the removal of a gate metal for measuring hydrogen and sodium activities in an aqueous solution. In the Bergveld device, the MOSFET was constructed without a gate metal so that when the transistor was placed in an aqueous solution, the oxide isulation layer would become hydrated and then, with the presence of impurities in the hydrated layer, would become ion selective. After hydration of the insulation layer of the MOSFET, it was suggested that the device could be used for ion activity measurement by immersing the device in the solution in question and then recording the changes of the conductivity of the device. The Bergveld device was considered to be inaccurate by Johnson et al. and others because the output of the Bergveld device varied over the lifespan of the device.
The Johnson et al. patent sought to improve the accuracy of the device by covering the exposed gate region with a chemically-selective membrane which responded selectively to the ion to be monitored. An external reference electrode was electrically attached by a wire to the source of the field-effect transistor to provide a means of referencing the field-effect transistor to the general ionic concentration of the solution to be monitored. One problem with the device as described in the Johnson et al. patent was that exposure of the gate region of the FET to ion concentrations through the ion selective membrane produced deleterious effects on the field-effect transistor. For example, if a gate region of a field-effect transistor is exposed to a test solution, the gate insulator may become easily contaminated by the solution. Certain contaminants, such as sodium ions, have a very high mobility in silicon dioxide. Thus, the resistance and other critical properties of the gate insulator are more than likely to be altered by the exposure of the device to a solution. As a result, the response of such a device may vary greatly with time and exposure in a manner similar to the Bergveld device.
U.S. Pat. No. 4,180,771, "Chemical Sensitive Field-Effect Transistor" by Guckel, issued Dec. 25, 1979, illustrates an attempt to improve on the devices designed by Johnson and Bergveld. In the Guckel patent, a device is described in which a chemically sensitive membrane is attached to the underside of a substrate of a field-effect transistor. The chemically sensitive membrane is not directly wired to either the source, gate, or drain regions of the field-effect transistor. Instead, the chemically sensitive layer is said to provide a potential to the substrate which effects the flow of current between the source and the drain in response to the concentration of the ion to be monitored. Guckel fixes the gate voltage with a battery and allows the potential of the substrate to vary. Guckel also maintains the voltage between the source and the drain at a fixed level.
A problem encountered by all of the devices described above includes the fact that electrical connections to and from the field-effect transistors through the use of wires reduce the integrity of the devices due the possible contamination of the FET at the point of electrical contact with each wire.